1. Field of the Invention
The invention relates to solar cells and somewhat more particularly to solar cells having a semiconductor body formed of amorphous silicon and which is applied to a substrate having a metal layer at least on one surface thereof and functioning as a back contact as well as a method of producing such solar cells.
2. Prior Art
The formation of solar cells comprised of amorphous silicon (sometimes referred to as a-Si) on insulating, but not necessarily transparent, substrates, such as glass, ceramic, or Kapton (a trade name for a commercially available polyimide film), is particularly significant because series-interconnected solar cells can be readily fabricated on such a substrate by a simple mask technique. In order to achieve this, a metal layer with a relatively low surface resistance (approximately 50 m Ohm with a 50 cm.sup.2 cell size) must first be applied to the substrate as a back contact.
It is known from the Conference Proceedings of the Fifteenth IEEE Photovoltaic Specialist Conference, Florida (1981) pages 922-927, especially FIG. 1, to utilize special steel discs as the substrate in producing a-Si solar cells.
However, aluminum would be particularly well suited as a back contact material because of its high conductivity and economical price. Decisive, with production by means of high frequency sputtering, is that the crystalline size of an aluminum layer so-produced is less than about 500 nm--which can be perceived a the reflective surface and is achieved by means of alloying with silicon--as well as that the resultant layer is thermally stable because of the subsequent a-Si:H deposition, which occurs at about 250.degree. C. Aluminum layers produced in this manner satisfy these requirements.
Aluminum could not heretofore be utilized as a substrate material for amorphous silicon cells because aluminum diffuses to a large degree into amorphous silicon already at temperatures of about 150.degree. C. and, thus, greatly deteriorates cell efficiency.